Wastewater from various origins is treated in wastewater treatment plants, in order to remove residues harmful for humans, animals, receiving waterbody and the nature in general. Typically, organic residue, phosphorus and nitrogen is removed from the wastewater. The purified water is then either returned to waterbody (such as a river, a lake or a sea) or reused as such (in agriculture or industry or as a raw water for drinking water purification). Phosphorus is, among other nutrients, responsible for eutrophication of natural waters. On the another hand, phosphorus is an essential material for fertiliser production as well as other chemical industry purposes, while at the same time it is becoming increasingly difficult to extract from nature and the reserves dry up. One possible phosphorus circulation flows through wastewater treatment process which makes it an interesting raw material source for recovered phosphorus. Existing recovery technologies focus strongly on side streams of the actual wastewater treatment processes i.e. wastewater sludge treatment processes or even burned sludge ash. However, the amount of phosphorus in the side stream wastewaters is typically rather low, thus making its recovery less profitable due to the small product streams obtained. Furthermore, recovery of phosphorus from ashes leads to a rather low yield of phosphorus, when compared to the amount of phosphorus entering the process, and as heavy metals concentrate in the ashes, recycling of the ashes is rather challenging. Moreover, very few techniques take into account recycling of possible precipitation agents and the effect such recycling has on the economic aspects of the process.
Document EP 2904892 presents a method for extracting phosphorus and nitrogen from a sludge. The method is suitable for sludges containing high amounts of phosphorus and nitrogen and does not directly result in a water stream that could be disposed of in a river or a sea. Document DE 10 2014 112108 discloses a traditional method where the phosphorus is precipitated in the sludge, the sludge is treated and phosphorus is recovered from the sludge. Document U.S. Pat. No. 4,565,680 presents a method from recovering phosphorus and aluminium from spent acid from an aluminium process. The method does not require using a precipitating agent, as aluminium is present in the spent acid.